LSI stands for Latent semantic indexing. It was Google's response to keyword stuffing. In this method, Google bots looked for the richness of content built around a few keywords. It demanded that your title tag should be bold and say, if you happen to mention world's deadliest snakes in your title tag, then Google bot would expect to find different synonyms of the words used in the title. It then collates different words present in the article and figures out what the article is actually all about. it is especially useful for bloggers and webmasters who wish to rank for their content. Caution however must be exercised in not to overuse certain keywords or Google penalizes not only that page but the reputation of the entire side resulting in lower page rank for the website.